1. Field of the Invention
This invention relates to a synchronous display control apparatus used in an intercommunication system for interchanging image information or plotting (writing) information among a plurality of communication terminals, and to the communication terminals used therein.
2. Related Arts
Nowadays, intercommunication systems have been proposed for exchanging image data or plotting data simultaneously with conversation among communication terminals interconnected over communication lines such as ISDN, telephone networks, private lines, or the like.
One such system is disclosed in Japanese Laid-open Patent Application No. 1-067061. According to a telewriting apparatus disclosed therein, plotting data on a transmitting terminal display, which is being drawn by a sender, is transmitted to an receiver and displayed on a receiving terminal display, so that both the parties can watch the same picture while talking to each other. A data which has been outputted by the transmitting terminal but has not yet reached the receiving terminal and another data which has been already received by the receiving terminal are displayed distinguishably from each other on the transmitting terminal display, so that the parties can be freed from inconvenience caused by the disagreement about the plotting data on the display.
Another intercommunication system, which is provided with a telephone set and a display unit, is disclosed in Japanese Laid-open Patent Application No. 2-084860. According to the system, various data processing programs can be used with the telephone set among a plurality of workstations or telecommunication terminals each provided with the multi-window system, so that intercommunication conference system can be realized.
However, these constructions are on the assumption that all the communication terminals process displaying operations at the same speed, and the following points are not taken into account:
1) The difference in the display processing performance between transmitting and receiving terminals. PA1 2) The reduction of the display processing performance by overload on the terminals. PA1 3) The time delay caused by transmitting data.
These may cause misunderstandings between the sender and the receiver because of the disagreement between the data displayed on the transmitting terminal and on the receiving terminal.